The present invention relates generally to the field of medical imaging by PET and by SPECT systems. The present invention relates in particular to PET and SPECT systems with simultaneous, single-photon transmission imaging for attenuation corrections.
Single photon emission computerized tomography (SPECT) and positron emission tomography (PET) are well known nuclear imaging systems in medicine. Generally, in nuclear imaging, a radioactive isotope is injected to, inhaled by or ingested by a patient. The isotope, provided as a radioactive-labeled pharmaceutical (radio-pharmaceutical) is chosen based on bio-kinetic properties that cause preferential uptake by different tissues. The gamma photons emitted by the radio-pharmaceutical are detected by radiation detectors outside the body, giving its spatial and uptake distribution within the body, with little trauma to the patient.
SPECT imaging is based on the detection of individual gamma rays emitted from the body, while PET imaging is based on the detection of gamma-ray pairs that are emitted in coincidence, in opposite directions, due to electron-positron annihilations. In both cases, data from the emitted photons is used to produce spatial images of the xe2x80x9cplace of birthxe2x80x9d of a detected photon and a measure of its energy. In PET, photon detectors also provide an indication of the time when a photon is detected.
An Anger gamma cameras generally comprises a scintillation crystal, which when struck by a photon emits light, an array of photomultiplier tubes (PMTs) arranged in a conventional hexagonal matrix, for giving the x-y position of the detected photon, various processing circuitry, and a processing unit. Solid state gamma cameras generally include an array of pixel sized detectors and may be based on one of a number of different technologies. Solid state cameras suitable for use in SPECT and/or PET imaging are described in PCT Application No. PCT/IL98/00462 filed on Sep. 24, 1998, now WO publication 00/17670 and PCT publication WO 98/23974, the disclosures of which are incorporated herein by reference.
Many SPECT and PET systems utilize one or two gamma cameras of either the Anger or solid state type to detect gamma rays.
SPECT and PET imaging couple conventional planar nuclear imaging techniques and tomographic reconstruction methods. Gamma cameras, arranged in a specific geometric configuration, are mounted on a gantry that rotates them around a patient, to acquire data from different angular views. Projection (or planar) data acquired from different views are reconstructed, using image reconstruction methods, to generate cross-sectional images of the internally distributed radio-pharmaceuticals. These images provide enhanced contrast and greater detail, when compared with planer images obtained with conventional nuclear imaging methods.
In general, it is desirous to have imaging systems that can be used both for PET and for SPECT, depending on the need.
A factor that influences the quality of imaging is non-uniform photon attenuation by intervening tissue, that is tissue through the which the gamma rays must pass before being detected by the gamma camera or cameras. Transmission scanning is a technique used to generate an attenuation map for correcting gamma images for non-uniform attenuation. In principle, gamma radiation from a known source, external to the tissue, is transmitted through the tissue to a corresponding scintillation detector. As in the cases of SPECT and PET, the external radiation source and the detector are rotated around the tissue, and transmission data from different angular views, coupled with tomographic reconstruction methods are used generate an attenuation map of the internal structure.
Two important points in generating an attenuation map are that transmission scanning must be performed for each patient individually, as patients differ in size and in internal structure and that transmission scanning should be performed on the same spatial registry as the PET or SPECT imaging, else it is difficult to correlate between the attenuation map and the PET or SPECT data.
Therefore, transmission scanning is generally performed simultaneously or concurrently with the PET or SPECT imaging, using the same detector system for the PET or SPECT imaging and for the transmission scanning. In many systems, use photons of different energies to differentiate between the transmission scanning photons and those of PET or SPECT imaging.
U.S. Pat. No. 5,900,636 to Nellemann, xe2x80x9cDual-Mode Gamma Camera system Utilizing Single-Photon Transmission Scanning for Attenuation Correction of PET Data,xe2x80x9d whose disclosure is incorporated herein by reference, describes a system of simultaneous PET of SPECT imaging and transmission scanning. FIG. 1 illustrates a view in a transverse (x-y) plane, in which two transmission point sources 30A and 31A are on the same side as two gamma detectors 10 and 11 in the transaxial (x) direction. Point sources 30A and 31A are mounted outside the fields of views (FOVs.) of detectors 10 and 11. Such mounting avoids blocking the detectors and reduces transmission self-contamination (where radiation from a point source strikes the detector near it). A transmission scan across the entire axial width of detectors 10 and 11 is performed at each angular stop about the z axis. The aggregate effect of these transmission scans with the illustrated placement of point sources 30A and 31A is a transmission FOV (of each transverse slice) represented by a circle 70. The emission field of view (in each transverse slice) is represented by circle 72.
In this device, the allowable width (in the transverse plane) of the fan beam generated by point sources 30A and 31A is limited by detectors 10 and 11. Thus, the transmission FOV 70 is defined by two boundaries, an outside boundary and an inside boundary. The outside boundary is defined by the outer edges of the transmission fan beams 68 and 69 at each of the angular stops about the z axis, while the inside boundary is defined by the circumference of a circle 76, which represents a blind spot in the attenuation map. In order to prevent the gap from resulting in incomplete data acquisition, the computer system (not shown) causes the examination table (not shown) to move vertically and horizontally relative to the z axis in dependence on the angular positions of the detectors 10 and 11 about the z axis, in order to provide full coverage of the object of interest.
An aspect of some preferred embodiments of the present invention relates to providing a PET or SPECT system with simultaneous, single-photon transmission scanning yielding complete coverage of all the tissue that is being examined, so that a complete tissue attenuation map is obtained, with no blind spots.
An aspect of some preferred embodiments of the present invention relates to providing a PET or SPECT system with simultaneous or sequential, single-photon transmission scanning wherein transmission scans can be obtained in one pass, rather than in a series of axial stops along the z axis.
An aspect of some preferred embodiments of the present invention relates to providing a PET system with simultaneous or sequential, single-photon transmission scanning wherein the transmission scanning can be shut off at will.
In one preferred embodiment of the invention, two gamma detectors are positioned facing each other, with a fan beam transmission source at the edge of one detector. The tissue to be examined is placed between them and the system is rotated about an axis. The axis is shifted towards the side of the line source. The tissue is completely within a rectangle bounded by the two detectors and the axis of rotation, is within the field of view of the fan beam line source. The source may be outside or inside the rectangle formed by (or the largest rectangle defined by) the gamma detectors.
In other preferred embodiments of the invention, two gamma cameras having slightly different transverse extents are utilized. One edge of the two detectors is so aligned with the other detector such that the other edge of the wider detector extends past a rectangle formed by the other three edges. A transmission source is placed at the edge of detector diagonally across from the extending edge. The extending edge is made to extend far enough such that the center of rotation of the two detectors is within a fan beam of radiation from the transmission source that is detected by the wider detector. The source may be outside or inside the rectangle formed by (or the largest rectangle defined by) the gamma detectors.
In other preferred embodiments of the invention, both effects are utilized and one detector extends past the other and the center of rotation is slightly displaced such that the center of rotation is within the fan beam.
An aspect of some preferred embodiments of the present invention relates to providing a transmission line source comprising multiple fan-beams so that scanning of many slices along the z direction can be obtained simultaneously. In preferred embodiments of this aspect, a radio-opaque rod, such as a tungsten rod, is used. Blind holes are drilled along the rod at equal distances, and a radioactive material of the desired properties is inserted into each hole. As such, the rod provides a linear array of point sources.
In a preferred embodiment of the invention, a radio-opaque plate is formed with slits spaced the same distance apart as the holes in the rod (and the sources) and having their long direction perpendicular to the axis of the rod. The slits are aligned with the sources such that a series of fan beams are formed. The plate which is thick enough to substantially limit the width of a beam such that the beams are limited in extent in the direction of the rod.
An aspect of some preferred embodiments of the present invention relates to providing a transmission source assembly wherein the transmission source can be shut off and turned on at will.
In some preferred embodiments of this aspect, a metal rod into which point sources have been inserted, as described above, is used. The metal rod is positioned on a sliding mechanism in a shielded box (for example a lead box or a tungsten box), with the edge of the rod, free of point sources, protruding from the side of the box. The top of the box is formed with open slits, as described above. When it is desired to shut off the point sources, the edge of the rod is pulled, so as to sufficiently misalign the point sources with the slits at the top of the box and to block radiation from exiting from the slits.
Alternatively, but less desirably, the rod itself is fixed, but the slits at the top of the box may be shut by putting a shielded cover on them.
There is thus provided, in accordance with a preferred embodiment of the invention, a nuclear imaging system comprising:
first and second radiation detectors, each comprising an imaging surface facing each other and each having an extent;
an radiation source, situated outside a space defined by a largest parallelepiped formed on two sides by said first and second detectors, which irradiates the second detector; and
an axis about which the first and second detectors and the radiation source rotate together;
wherein the field of view of the radiation source, defined by lines connecting the external source and the edges of the second detector, encompass the axis of rotation.
In a preferred embodiment of the invention, the first and second radiation detector have different transverse extents, one edge of the two detectors is aligned with each other such that the other edge of the wider detector extends past the parallelepiped and wherein the radiation source is placed near the edge of the short detector diagonally across from the extending edge.
In a preferred embodiment of the invention, the first and second radiation detector have different transverse extents, both edges of a short detector being outside said parallelepiped, wherein the radiation source is placed near an edge of the short detector and diagonally opposed edge of the wider detector extends past the parallelepiped by an amount sufficient such that the axis of rotation is within the field of view.
Preferably, the radiation source comprises a line of sources extending along the edge of the shorter detector.
Preferably, the irradiation source comprises a plurality of point sources collimated to produce fan beams each defining a plane perpendicular to the axis of rotation.
Preferably, the extending edge is made to extend far enough such that the center of rotation of the two detectors is within a fan beam of radiation from the transmission source that is detected by the wider detector.
Preferably, the axis of rotation is substantially at the center of the parallelepiped or of a rectangle defined by edges of the shorter detector and the edge of the longer detector aligned with the edge of the shorter detector.
Alternatively or additionally, the axis of rotation is displaced such that the center of rotation is within the field of view of the line source. Preferably,the axis of rotation is displaced in the direction of the aligned edges, from the center of a rectangle defined by edges of the shorter detector and the edge of the longer detector aligned with the edge of the shorter detector. Alternatively or additionally, the axis of rotation is displaced in the direction of the detector farther from the source.
In a preferred embodiment of the invention, the first and second detectors have substantially the same extent and are aligned with each other at one side of the parallelepiped and wherein the axis of rotation is displaced from the center of the parallelepiped.
In a preferred embodiment of the invention, the first and second detectors have different extents and wherein the edges of the detectors are aligned with each other on one side of the parallelepiped, the source is placed near the opposite edge of the smaller detector and the axis of rotation is displaced from the center of the parallelepiped.
Preferably, the axis of rotation is displaced toward the open side of the parallelepiped at which the radiation source is situated. Alternatively, the axis of rotation is displaced in the direction of detector farther from the source.
Preferably, the radiation source comprises a line of sources extending along the edge of the shorter detector. Preferably, the irradiation source comprises a plurality of point sources collimated to produce fan beams each defining a plane perpendicular to the axis of rotation.
There is further provided, in accordance with a preferred embodiment of the invention, a nuclear imaging system comprising:
first and second radiation detectors, each comprising an imaging surface facing each other and each having an extent;
an radiation source, situated inside a space defined by a largest parallelepiped formed on two sides by said first and second detectors, which irradiates the second detector; and
an axis about which the first and second detectors and the radiation source rotate together;
wherein the field of view of the radiation source, defined by lines connecting the external source and the edges of the second detector, encompass the axis of rotation and wherein the field of view of the radiation source does not encompass the center of the parallelepiped.
In a preferred embodiment of the invention, the first and second radiation detector have different transverse extents, one edge of the two detectors is aligned with each other such that the other edge of the wider detector extends past the parallelepiped and wherein the radiation source is placed near the edge of the short detector diagonally across from the extending edge. Preferably, the radiation source comprises a line of sources extending along the edge of the shorter detector. Preferably, the irradiation source comprises a plurality of point sources collimated to produce fan beams each defining a plane perpendicular to the axis of rotation. Preferably, the extending edge is made to extend far enough such that the center of rotation of the two detectors is within a fan beam of radiation from the transmission source that is detected by the wider detector.
In a preferred embodiment of the invention, the axis of rotation is substantially at the center of the parallelepiped. Alternatively, the axis of rotation is displaced from the center of the parallelepiped, such that the center of rotation is within the field of view of the line source. Preferably, the axis of rotation is displaced in the direction of the aligned edges. Alternatively or additionally, the axis of rotation is displaced in the direction of the detector farther from the source.
In a preferred embodiment of the invention, the first and second detectors have substantially the same extent and are aligned with each other at one side of the parallelepiped and wherein the axis of rotation is displaced from the center of the parallelepiped. Preferably, the axis of rotation is displaced toward the open side of the parallelepiped at which the radiation source is situated. Alternatively or additionally, the axis of rotation is displaced in the direction of detector farther from the source.
Preferably, the radiation source comprises a line of sources extending along the edge of the shorter detector. Preferably, the irradiation source comprises a plurality of point sources collimated to produce fan beams each defining a plane perpendicular to the axis of rotation.
There is further provided, in accordance with a preferred embodiment of the invention a radiation source comprising:
a source of radiation;
a plate having an aperture formed therein, said aperture facing the source of radiation; and
means for moving the source when no radiation is desired on the side of the plate away from the source.
In a preferred embodiment of the invention, the plate is a flat plate. Preferably, the plate has a thickness that is greater than the smallest dimension of the aperture. Preferably, the thickness is more than five or ten as large as the smallest dimension of the aperture.
Preferably, the aperture has a slit shape, such that the radiation exiting the slit forms a collimated fan beam.
There is further provided, in accordance with a preferred embodiment of the invention, a radiation source comprising:
a plurality of individual sources of radiation;
a plate having an apertures formed therein, each said aperture facing a respective individual source of radiation; and
means for moving the sources such that they do not face the apertures when no radiation is desired on the side of the plate away from the source.
Preferably, the plate is a flat plate.
Preferably, the means for moving displaces the sources so that they are situated between the apertures when no radiation is desired. Alternatively or additionally, the means for moving rotates the sources so that they do not face in the direction of the plate. Preferably, the plate has a thickness that is greater than the smallest dimension of the aperture. Preferably, the thickness is more than five or ten as large as the smallest dimension of the aperture.
Preferably, the aperture has a slit shape, such that the radiation exiting the slit forms a collimated fan beam.
There is further provided, in accordance with a preferred embodiment of the invention, a radiation source comprising:
a plurality of individual sources of radiation; and
a plate having an apertures formed therein, each said aperture facing a respective individual source of radiation,
wherein the plate has a thickness that is greater than the smallest dimension of the aperture.
Preferably, the plate is a flat plate.
Preferably, the means for moving displaces the sources so that they are situated between the apertures when no radiation is desired. Alternatively or additionally, the means for moving rotates the sources so that they do not face in the direction of the plate. Preferably, the plate has a thickness that is greater than the smallest dimension of the aperture. Preferably, the thickness is more than five or ten as large as the smallest dimension of the aperture.
Preferably, the aperture has a slit shape, such that the radiation exiting the slit forms a collimated fan beam.